1. Field of the Invention
The present invention generally relates to fuses and more particularly to a programmable magnetic fuse which has more than two states.
2. Description of the Related Art
Conventional systems utilize fuses in semiconductor chips to provide redundancy, electrical chip identification and customization of function. For designs having three (or more) layers of wiring, the fuses are typically formed from a segment of one of the wiring layers, e.g. the xe2x80x9clast metalxe2x80x9d or xe2x80x9clast metal minus onexe2x80x9d wiring layer. Fusing (i.e. deletion of a segment of metal fuse line) is accomplished by exposing the segment to a short, high intensity pulse of xe2x80x9clightxe2x80x9d from an infra-red laser. The metal line absorbs energy, superheats, melts and expands, and ruptures any overlaying passivation. The molten metal then boils, vaporizes or explodes out of its oxide surroundings, disrupting line continuity and causing high electrical resistance. A xe2x80x9csensingxe2x80x9d circuit is used to detect fuse segment resistance. Sense circuits can be designed to xe2x80x9cunderstandxe2x80x9d that fusing has occurred when line resistance increases or line resistance decreases.
Because of line-to-line coupling which slows signal propagation, there is a trend for the Inter-Level Dielectric (ILD) to be comprised of polyimide nanofoams or porous glass (Zero Gel is an example). These dielectrics are of a class characterized by having low dielectric constants (K). They are not solid materials and will collapse under standard laser fuse blow conditions, leading to unreliability and possibly yield degradation.
Also, to improve signal propagation, high conductance materials such as copper are being used as conductors. More specifically, resistance is reduced by using copper interconnect wiring. However, if copper lines are blown and left exposed, they easily corrode resulting in reliability exposures.
However, conventional laser blowing (or even electrical blowing) may damage the ILD, causing it to collapse and change its dielectric constant. The laser fusing could also release particles of copper into the porous films, presenting potential reliability concerns and possibly comprising the basic local structural integrity of the chip.
Therefore, there is a need for a new type of fuse structure which can be blown (e.g., opened) without suffering the reliability problems associated with conventional fuses.
Therefore, it is an object of the present invention to provide a structure and method for a programmable circuit that includes a magnetic device having a reluctance which is alterable. The programmable circuit includes the magnetic material and a sensing circuit for detecting the reluctance. The reluctance includes a magnetic field aligned in a first direction. The programmable circuit further includes a plurality of magnetic devices having different reluctances. The first devices of the magnetic devices have a first magnetic field aligned in a first direction and second devices of the magnetic devices have a second magnetic field aligned in a second direction, different than the first direction. The third devices of the magnetic devices have no magnetic field. The magnetic devices are tristate fuses.
A method of programming magnetic devices includes aligning magnetic fields of the magnetic devices in a first direction, selectively aligning magnetic fields of first devices of the magnetic devices in a second direction and sensing a magnetic state of the magnetic devices. The aligning process further includes heating and cooling the magnetic devices in a first magnetic field and the selectively aligning includes heating and cooling the first devices in a second magnetic field. The second magnetic field could have a directional characteristic different than the first magnetic field and the second magnetic field could have a strength less than that of the first magnetic field. The heating could be selective application of laser energy.
The method may also selectively neutralize magnetic fields of the second devices. The neutralizing includes heating and cooling the second magnetic devices in a magnetic field equal to and opposite the Earth""s magnetic field. The neutralizing may alternatively include removing the second devices.
This structure and process is superior to conventional fuses because low level energy magnetic and laser sources are utilized to program the device instead of conventional higher power laser sources. The energy level utilized by the invention does not cause the fuse material to explode or boil as conventional fusing processes do. Therefore, programming (or blowing) the inventive device does not damage the surrounding dielectric material. Further, since the magnetic material does not explode and possibly extend above the dielectric, it will not corrode or detract from the reliability of the remaining circuit.